The photocatalytic properties of the semiconducting titanium dioxide material result from the promotion of electrons from the valence band to the conduction band under the influence of ultraviolet (UV) and near-UV radiation. The reactive electron-hole pairs that are created migrate to the surface of the titanium dioxide particles where the holes oxidize adsorbed water to produce reactive hydroxyl radicals and the electrons reduce adsorbed oxygen to produce superoxide radicals, both of which can degrade NOx and volatile organic compounds (VOCs) in the air. In view of these properties, photocatalytic titanium dioxide has been employed in coatings (both encased and unencased) and the like to remove pollutants from the air. Such coatings may also have the advantage of being self-cleaning since soil (grease, mildew, mold, algae, bacteria, etc.) is also oxidized on the surface.
In many applications, it is desirable for the titanium dioxide coating to be transparent in order to maintain the original appearance of the substrate (e.g., ceramic tile, paving block, brick, stone, marble siding surgical instruments used during medical procedures, solar cells, woven and non-woven fabrics of natural or synthetic fibers, etc.) or its original transparency (e.g., window glass, car windshield, surgical instruments used for viewing etc.). Titanium dioxide colloidal sols have proven to be useful precursor materials for forming such transparent and reactive coatings.
A stable alkaline titanium dioxide sol can be established at a pH above 11.30. The sol can be used for forming coatings that can be applied to buildings, concrete surfaces and roadways. However, the sol often at times has a strong and irritating pungent “ammonium type” smell and can be flammable under certain conditions. Thus, it is difficult to use such a sol without incurring the expense and difficulty of putting on personal protective equipment and the expense of employing extraction technology for removing residual amounts of sol from areas (such as ground soil) adjacent the substrate to be treated. In addition, the substrates may adversely react with the high pH peptizing agents that exist in the sol.
It is desirable to neutralize the alkaline titanium dioxide sol so that the sol will become virtually odorless and non-flammable-thereby overcoming many of the above-noted deficiencies, and allowing such a sol to be applied as an environmentally friendly photoactive water based surface coating. However, as the pH of the alkaline titanium dioxide sol is reduced the colloidal system typically agglomerates and becomes unstable and may even collapse. Such agglomeration is irreversible, that is, even if the solution is re-adjusted back to a high pH, colloidal stability is not returned. It is therefore desirable and long sought after in the art to provide stable neutral sols comprising photocatalytic titanium dioxide which are reactive, neutral and transparent. It is also desirable for such reactive, neutral and transparent photocatlytically active titanium dioxide sols to be stable over an extended period of time and also maintain photoactivity over a period of time and at a rate of activity that is above what is currently commercially available. It is well known to one of ordinary skill in the art that the creation of a stable, transparent and neutral TiO2 sol is difficult (and has long been sought in the industry) because of the natural tendency of TiO2 to flocculate between pH values of about 4 to about 10 due to the absence of any electrostatic stabilization occurring at neutral (or near neutral) pH. Furthermore, it is difficult to identify appropriate or effective molecules which can act as steric stabilizers in TiO2 sols due to the extremely small particle size of the TiO2. As effective steric stabilizers are generally larger molecules, one of ordinary skill in the art has found it difficult to identify appropriate steric stabilizers for use. It is also desirable for such reactive, neutral and transparent photocatalytically active titanium dioxide sols to exhibit anti-bacterial/anti-microbial activity. It is further desirable that the presently disclosed and claimed inventive concept(s) provide novel methods for preparing such reactive, neutral, stable and transparent sols which are readily implemented on a commercial scale.